ISO 16840-12:2021

FINAL
INTERNATIONAL ISO/FDIS
DRAFT
STANDARD 16840-12
ISO/TC 173/SC 1
Wheelchair seating —
Secretariat: SABS
Voting begins on:
Part 12:
2021-09-09
Envelopment and immersion
Voting terminates on:
characterization of seat cushions
2021-11-04
using a dual semispherical indenter
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OF ANY RELEVANT PATENT RIGHTS OF WHICH
THEY ARE AWARE AND TO PROVIDE SUPPOR TING
DOCUMENTATION.
IN ADDITION TO THEIR EVALUATION AS
Reference number
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO-
ISO/FDIS 16840-12:2021(E)
LOGICAL, COMMERCIAL AND USER PURPOSES,
DRAFT INTERNATIONAL STANDARDS MAY ON
OCCASION HAVE TO BE CONSIDERED IN THE
LIGHT OF THEIR POTENTIAL TO BECOME STAN-
DARDS TO WHICH REFERENCE MAY BE MADE IN
©
NATIONAL REGULATIONS. ISO 2021

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ISO/FDIS 16840-12:2021(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2021
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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ii © ISO 2021 – All rights reserved

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ISO/FDIS 16840-12:2021(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Indenter construction . 2
4.1 Materials . 2
4.2 Tolerances and finishes . 2
4.3 Indenter elements . 2
4.3.1 Trochanter bar . 2
4.3.2 Trochanter blocks . 3
4.3.3 Semispherical indenter half . 4
4.4 Indenter assembly . 5
4.5 Sensor locations . 6
4.5.1 General. 6
4.5.2 Elevation a . 7
4.5.3 Elevation b . 7
4.5.4 Elevation c . 7
4.5.5 Elevation d . 7
5 Instrumentation hardware . 8
5.1 Sensors . 8
5.2 Timer . 8
5.3 Loading equipment . 8
5.4 Rigid board . 8
6 Preparation of test cushion . 8
6.1 Cushion set up . 8
6.2 Preconditioning the cushion to the test environment . 9
7 Envelopment test method . 9
7.1 Rationale. 9
7.2 Method . 9
7.2.1 Setting the datum height for the indenter . 9
7.2.2 Pre-conditioning the cushion . 9
7.2.3 Seat cushion thickness measurement . 9
7.2.4 Load application and data collection .10
7.3 Calculations .11
7.3.1 Envelopment .11
7.3.2 Immersion .12
8 Test report .12
Annex A (informative) Estimate of measurement uncertainty .14
Bibliography .15
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ISO/FDIS 16840-12:2021(E)

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso .org/
iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 173, Assistive products, Subcommittee
SC 1, Wheelchairs.
This first edition of ISO 16840-12 cancels and replaces ISO/TS 16840-12:2015, which has been
technically revised.
The main changes compared to the previous edition are as follows:
— clarification of technical ambiguities;
— the removal of a gel cap to protect the pressure sensors;
— removal of former Annex A (which addressed method adaptations for cushions that utilize
offloading);
— addition of a new Annex A giving guidance on the estimation of uncertainty.
A list of all parts in the ISO 16840 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
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ISO/FDIS 16840-12:2021(E)

Introduction
This document provides details of test equipment (an ‘indenter’), and a method, for the measurement
of ‘performance’ of a wheelchair cushion intended to use immersion and envelopment to reduce local
areas of pressure (by effectively supporting more tissue). The cushioning performance of a support
surface is defined by its ability to immerse and envelop. Envelopment is maximized when the contact
area between the support surface and the body part increases which thereby reduces the pressure
across the weight bearing surface. The method as presented in this document is intended to quantify
envelopment and immersion of the body by the cushion. Alternatively, the test method and resulting
data can provide an indication of other cushion construction and seating and positioning strategies,
such as strategic pressure offloading.
Issues related to the use of devices that measure interface pressure between the body and a support
surface (e.g. a pressure mapping system) have led to the use of an instrumented indenter, which has
fewer sensors, but sensors that are high quality and exhibit repeatability and accuracy and thus
reliability.
In this test, the distribution of pressure across the surface of the indenter is evaluated with multiple
indenter sizes and loads. The pressure sensors within the indenter are positioned at multiple elevations
along the buttock form, to simulate and record the pressures experienced at the ischial tuberosities
of the pelvis, the greater trochanters, and at positions along the curve of the buttocks between those
anatomical markers. The review of the values at these various positions, and the variation, or lack of
variation, in the readings, is an indication of the mechanics of the cushion’s interaction with the body
and ability to envelop and protect the tissues.
EXAMPLE A fluid filled cushion with the ability to transfer material between cells can have the potential to
distribute the load to maintain consistent interface pressure regardless of the depth at which the measurement
is taken, as opposed to a foam or other homogeneous surface that behaves more like a spring, in that the greater
the depth of immersion, the greater the interface pressure.
The accommodation of the cushion to the changes in indenter size and load are representative of
the changes in size and mass of the occupant that can occur in the life of a user or between different
users. The indenters are sized to represent changes in size and shape as a user of an approximately
410 mm width cushion gains weight, and the size of the buttocks increases. The overall width of the
indenters stays the same, but the size of the semi spheres changes. The loads used in this document are
th
approximate to the 50 percentile user and are not intended to characterize envelopment or immersion
under higher loading conditions, nor to assess the weight capacity of a cushion.
This document describes test methods that might not be appropriate for all cushions, and therefore the
tester is responsible for determining which, if any, are appropriate for their cushion construction and
use.
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FINAL DRAFT INTERNATIONAL STANDARD ISO/FDIS 16840-12:2021(E)
Wheelchair seating —
Part 12:
Envelopment and immersion characterization of seat
cushions using a dual semispherical indenter
1 Scope
This document specifies apparatus, test methods, and disclosure requirements for characterization of
wheelchair seat cushion immersion and envelopment properties using indenters instrumented with
pressure sensors.
This document expands the characterization of products intended to manage tissue integrity
(ISO 16840-2) and provides a standardized indenter for other wheelchair seating tests.
It does not provide information specific to cushion performance for a particular individual user, nor is it
intended to characterize envelopment or immersion under higher loading conditions, nor to assess the
weight capacity of a cushion.
This document includes a method that is specific to 220 mm and 255 mm indenters. Dimensions are
provided for a 380 mm indenter to allow for extension of the method to larger patient simulation.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 1302, Geometrical Product Specifications (GPS) — Indication of surface texture in technical product
documentation
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
clinical offloading
reduction, removal, or transfer of pressure from one area of the body to another
EXAMPLE Reduction of pressure under the ischial tuberosities, but increased pressure on the thighs or
other parts of the seated body.
3.2
cushion envelopment
ability to conform around a shape
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ISO/FDIS 16840-12:2021(E)

3.3
cushion immersion
depth from an uppermost plane to which a body penetrates
3.4
indenter
shaped item designed to simulate the shape of the buttocks area of the human body
3.5
indenter base point
lowest point on the curved surface of each of the two halves of the indenter
Note 1 to entry: The base points correspond with the ischial tuberosities on the human pelvis.
3.6
indenter sensor elevation
location of sensors relative to the indenter base points (3.5)
4 Indenter construction
4.1 Materials
The indenter shall be constructed from hardwood or similar material that is sufficiently rigid so as not
to deform when subjected to the forces required for simulating the application of a human body mass
to surfaces. The indenter material shall be treated (if necessary) to minimize the effects of moisture,
and shall not be adversely affected by normal laboratory testing conditions (0 °C to 35 °C; 25 % to 75 %
relative humidity). Surface finish shall be at least N7, in accordance with ISO 1302, approximate average
surface roughness.
4.2 Tolerances and finishes
Unless otherwise stated, all dimensions in Clause 4 shall be ±0,5 mm and all edges and corners shall be
finished with a minimum 5 mm radius.
4.3 Indenter elements
4.3.1 Trochanter bar
A 375 mm x 50 mm x 20 mm bar with a series of openings as shown in Figure 1. These openings are for
mounting the indenter halves and trochanter blocks as well as for routing of wires.
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ISO/FDIS 16840-12:2021(E)

Dimensions in millimetres

a
THRU ALL.
b
Typ. THRU ALL.
NOTE d is the diameter of the pressure sensor selected to meet requirements of 5.1. See 4.1 and 4.2 for
materials, tolerances, and finishes.
Figure 1 — Trochanter bar construction
4.3.2 Trochanter blocks
50 mm × 25 mm × 19 mm blocks as shown in Figure 2, which emulate the trochanters. Each trochanter
bar shall have two trochanter blocks that house the sensors.
NOTE Trochanter bar and blocks are not required on the 380 mm indenter.
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ISO/FDIS 16840-12:2021(E)

Dimensions in millimetres

a
THRU ALL.
d is the diameter of the pressure sensor selected to meet the requirements of 5.1. Sensor location depth e shall be
such that the sensing surface is flush with the indenter surface. See 4.1 and 4.2 for materials, tolerances and finishes.
Figure 2 — Trochanter block construction
4.3.3 Semispherical indenter half
The semispherical indenter half is a portion of a sphere (see Figure 3). The sphere diameter defines the
size of the indenter (e.g. a 220 mm indenter has two semispherical halves sectioned from a 220 mm
diameter sphere) (see Table 1). Each half has eight sensor locations, as descr
...